Since those are two less common works by Hindemith, I'd suggest listening to them first. Blomstedt/San Francisco recorded "Harmonie der Welt" for Decca, presumably with Floyd Cooley on tuba (?). It's a fairly complicated and densely polyphonic work. "Symphonia Sernea" is even more rare, but I know that Chandos made a good recording of it with - I think - the Czech Phil. There are probably some performances of both works on Youtube.
Usually the hardest part of playing in a symphony orchestra is just knowing when to come in, and gauging how to make it fit (or stand out, if that's called for). Listening to the tunes first really helps with that.